Polymer electrolyte membrane is a novel functional material used in secondary cells, fuel cells, and the like. In general, polymer electrolyte membranes are classified into perfluorinated polymer electrolyte membranes and hydrocarbon polymer electrolyte membranes. Perfluorinated polymer electrolyte membranes are commonly used due to the advantages that they have chemical stability because of the strong carbon-fluorine (C—F) bond and the shielding effect characteristic of the fluorine atom, have superior mechanical properties and, particularly, have superior conductivity as proton exchange membrane. Currently, the most widely used commercially-available polymer electrolyte membrane material is Nafion™ (perfluorinated sulfonic acid polymer) of DuPont, U.S.A. Nafion™, a representative example of commercial proton exchange membrane, is widely used because of superior ion conductivity, chemical stability, ion selectivity, etc. Despite the superior performances, production of perfluorinated polymer electrolyte membranes is rather complicated regarding raw materials and manufacturing process, making it less applicable to industry. Besides, the fluorine atoms contained in the polymer main chain may cause environmental pollution and also generate some toxic substances. Further, they have a high tendency of methanol crossover, or permeation of methanol through the polymer membrane, and polymer membrane capacity is greatly reduced at 80° C. or above.
Numerous researches have been performed to prepare polymer electrolyte membranes through partial modification of Nafion™ or polyaryl-based polymers. However, the polymer electrolyte membranes obtained by polymer modification have the problems of low hydration stability, low proton conductivity, and the like.
Typically, sulfonate-substituted polymer electrolyte membranes are produced by direct copolymerization or post-sulfonation. Both of them make use of nucleophilic aromatic substitution. Direct copolymerization is advantageous in that control of the sulfonate groups introduced to the polymer main chain is easier than post-sulfonation. However, monomers having sulfonate groups are needed to produce polymer electrolyte membranes through direct copolymerization. Typical examples of monomers having sulfonate groups are 3,3′-disulfonated-4,4′-dichlorodiphenylsulfone (SDCDPS), 3,3′-disulfonated-4,4′-difluorodiphenylsulfone (SDFDPS), 5,5′-carbonylbis(2-fluorobenzenesulfonate), etc. However, considering the significant values of polymers having sulfonate groups from the industrial point of view, monomers available for production of such polymers are still far too scarce.
Accordingly, there is an urgent need for the development of new sulfonate-containing materials useful as polymer electrolyte membrane materials.